1. Field of the Invention
The present invention relates to a wafer aligning apparatus for aligning a plurality of wafers, and, in particular, to a wafer aligning apparatus which is capable of re-aligning a plurality of wafers at different pitches.
2. Description of the Related Art
Hitherto, semiconductor wafers have been coated, for example, by oxide film deposition at high temperature, in the following way.
As illustrated in FIG. 11, 25 wafers 2 are accommodated, for example, in a cassette 1. Here, the wafers 2 are accommodated at the same pitch, with their surfaces 2a each facing the same direction. The worker holds the cassette 1 in his hand, contacts a vacuum tweezer 3 to the back side 2b of one wafer 2, which results in the back side being grasped and retained. With the wafer 2 placed on an aligning boat 4, the vacuum tweezer 3 is moved away from the wafer 2. In this way, the twenty-five wafers 2, accommodated in the cassette 1, are transferred, one at a time, to the aligning boat 4. After all the wafers 2 are transferred, the aligning boat 4 is moved to a film forming apparatus 5, where the wafers 2 come into contact with a reacting gas at a high temperature, which forms a film on the wafers 2.
After the coating, the aligning boat 4 is drawn out of the film forming apparatus 5 for the wafers 2 to cool naturally. Then, the worker uses the vacuum tweezer again to pull out, one at a time, the wafers 2 from the aligning boat 4, and transfers them into the cassette 1.
As shown in FIG. 12, the reacting gas is supplied to the film forming apparatus 5 from an inlet 5a. After coming into contact and reacting with the surface of each of the wafers 2 placed on the aligning boat 4 in a tubular film forming apparatus body 5b, the gas is discharged from an outlet 5c. Here, the adjacent wafers 2 must be spaced at an interval greater than a predetermined interval to make uniform the density of the reacting gas which comes into contact with each of the surfaces of the wafers 2. That is, when the adjacent wafers 2 are not sufficiently spaced apart, the reacting gas cannot flow smoothly between these wafers, thus making it difficult to make uniform the coating on each of the wafers 2 throughout its surface.
When each of the wafers 2 is spaced further apart and aligned, the coating capability of the film forming apparatus is reduced because of a reduced number of wafers 2 which can be accommodated therein.
As illustrated in FIG. 12, measures have been taken, conventionally, to counter this problem by aligning the wafers 2 such that each of the adjacent wafers face each other, with pitch P1 set, for example, at about 9.52 mm, when the surfaces 2a of each of the adjacent wafers 2 face each other, on the one hand, and pitch P2 set, for example, at 1/2 the pitch of P1 (4.76 mm), when the back surfaces 2b thereof face each other, on the other. This prevents reduced coating performance of the film forming apparatus 5 from occurring because the reacting gas flows smoothly at each of the surfaces 2a of the wafers 2 which results in uniform coating, and the film forming apparatus body 5b is able to accommodate a large number of wafers 2.
However, in the aligning method illustrated in FIG. 12, when the worker transfers the wafers 2 from the cassette 1 to the aligning boat 4, and vice versa, he must take out the wafers 2, one by one, with the vacuum tweezer 3, and place the wafers 2 such that they alternatingly face opposite each other, in the cassette 1. This requires much time and effort, and may give rise to scratched wafers 2 by the use of the vacuum tweezer 3, and the possibility of wafers 2 having foreign substances adhered thereto.